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Lecture 16 Membrane Transport Active transport. Where would you find active transport?. interface with the environment…. maintain cell volume control internal environment signaling….Ca ++ gradient. Characteristics of a Transporter. Saturability…characterized by K M and V max

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Lecture 16

Membrane Transport

Active transport

Where would you find active transport?

  • interface with the environment….

  • maintain cell volume

  • control internal environment

  • signaling….Ca++ gradient


Characteristics of a Transporter

  • Saturability…characterized by KM and Vmax

  • Stereospecificity..or specificity unrelared to biophysical characteristics

  • Higher rate than expected from oil/water partition coef.



Vmax

1

Km = 1 mM

0.8

Km = 10 mM

0.6

0.4

0.2

0

0

10

20

30

40

50

[s], mM

Michaelis-Menten equation for enzyme/transport reactions is very similar to the Langmuir isotherm

A “simple explanation” says that the rate of reaction should be proportional to the occupancy of the binding site as long as Vmax is constant.



The Lac permease functional cycle,

an example of coupled transport

Note: the proton is always taken up first, but is released at last, which ensures strict coupling of transport without H+ leakage

from Abramson et al. 2003


energy in gradient:

Example:

Na+-glucose symport: stoichiometry of 2:1

at equilibrium: Δμglu= -2ΔμNa


Aspartate Transporter:

Na+ - dependent transport of aspartate

(from Boudker et al., Nature 2007)


apical

Na-K ATPase = the primary active transport, generates concentration gradients of Na+ and K+

utilizing ATP

Tight junction

Na-Glucose co-transporter, utilizes Na+ gradient as a secondary energy source

GLUT

Glucose diffusion facilitator (no energy consumed, passive transport)

H2O

basolateral


ATPases that couple splitting of ATP with ion motion across the membrane

ATP synthase

(works in reverse)

pump only protons


During contraction of the striated and cardiac muscle, Ca the membrane2+ is released into the cytoplasm, but during the relaxation phase it is actively pumped back into SR. Ca2+ ATPae (SERCA) constitutes >80% of total integral protein in SR.


Muscle Ca the membrane2+ pump (SERCA)

High-affinity state

open inside

Low-affinity state

open outside


The activity of SERCA, especially in the heart is regulated by Phospholamban, a small (single-pass) transmembrane protein. Phosphorylation of phospholamban by PkA removes its inhibitory action and increases the activity of SERCA by an order of magnitude.

The activity of plasma membrane Ca2+ pump (p-class) is regulated by calmodulin, which acts as a sensor of Ca concentration. Elevated Ca2+ binds to calmodulin, which in turn causes allosteric activation of the Ca2+ pump.


Post-Alberts Cycle for the Na+/K+ ATPase by Phospholamban, a small (single-pass) transmembrane protein. Phosphorylation of phospholamban by PkA removes its inhibitory action and increases the activity of SERCA by an order of magnitude.





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